The present invention relates to electrochromic assemblies having controllable light transmittance, their production and their use.
The transparency of windows of vehicles in respect of electromagnetic radiation has hitherto not been able to be regulated. Phototropic glasses have hitherto been used only as glass in spectacles and have only a relatively small change in the transmission. Windows of buildings have hitherto been darkened by means of curtains, shutters, roller blinds or other movable mechanical elements. Electrochromic devices can thus be employed in a wide variety of ways. A brief overview of examples is as follows:
1. Vehicle glazing (windows or sunroofs of automobiles)
An electrochromic device is suitable as protection against sun or dazzling in motor vehicles. Front, side and rear windows or glass roofs can be included. The degree of darkening can be matched zone-wise and steplessly to the needs of the driver depending on the position of the sun and the immediate driving situation. Integration into a computer-controlled regulating system is possible. A combination of an active element with a laminated glass unit is likewise possible, for example application of a film system to the safety glass.
The transmittance of the windows can be controlled manually or automatically, which can be used for effective protection against dazzling during night driving, automatic adjustment of the level of brightness on driving into and out of tunnels and multistorey car parks and for protection against forced entry and theft when the vehicle is parked by preventing a view into the interior of the vehicle. Excessive heating of the interior in summer, particularly when the vehicle is parked can be prevented (cf. EP-A 0 272 428).
2. Glazing of buildings (electrochromic window)
In buildings, electrochromic assemblies are suitable for darkening side windows and skylights of buildings, living areas, workrooms or greenhouses as controllable sun protection (visible spectral region) and heat protection (IR region) and also for protection of the eyes (visible spectral region). For protection against break-ins, glazing of bank counters or shop windows can be darkened at the press of a button. Glass doors can automatically be made visible on the approach of persons in order to avoid injury. The ability to generate virtually all colours also makes it possible to incorporate the glazing architecturally into the facade of a building. The energy consumption for controlling the transparency of a large area of window is low, particularly when the memory effect of the system can be exploited and energy is only consumed in the switching phase. A combination with heat-protection glazing (K glass) is very well suited to achieving dynamic control of the sunlight shining through a window ("smart window"). Thus, an electrochromic system can contribute to regulating and limiting the energy required for air conditioning of buildings.
The power supply to the system can also be achieved by means of solar modules. A light-sensitive sensor can determine the intensity of the sunlight and thus control the light transmittance.
3. Display Elements
The ability to produce attractive colours and display any desired contours, e.g. letters, numbers, signs and symbols (able to be produced by appropriate structuring techniques) on a large area provides advertising with an interesting medium. Decorative and informative effects are readily possible.
Apart from the possibility of locating the system between panes of glass, there is also the alternative of using two or even only one transparent plastic film as support. This makes it possible to achieve placard-like advertizing materials with changeable information.
Electrochromic devices can be used for small display elements such as faces of watches and clocks or measuring instruments, displays for a wide variety of applications and for large display elements such as traffic signs, advertizing columns, information displays at railway stations and airports or for providing parking directions. Use as variable delineation system (marking of boundaries etc. on playing areas) in sports halls is likewise possible.
They can be used wherever information is to be made visible.
4. Optics
In optics, electrochromic systems can be used either in combination with glasses, lenses and filters of other optical instruments as well as sole active components. Use as fade-over protection for optical detection systems is likewise possible. The system is likewise suitable as a controllable filter system in photographic processes.
5. Mirrors
An electrochromic device can also be used as a dimmable mirror, e.g. in an automobile as external or rear-view mirror, which can be darkened by application of an electric potential and thus prevents dazzling by the headlights of other vehicles (cf., for example, U.S. Pat. No. 3,280,702, U.S. Pat. No. 4,902,108, EP-A 0 435 689, U.S. Pat. No. 5,140,455). A disadvantage of systems of the prior art (solution systems) is the colour in homogeneity after prolonged operation (segregation), particularly in the case of large mirrors (e.g. mirrors of goods vehicles). Increasing the viscosity of the solution system by addition of polymeric thickeners has been described (e.g. U.S. Pat. No. 4,902,108).
6. EMI Shielding
An electrochromic device can also be used as a variable filter element for modulating electromagnetic radiation in certain wavelength ranges.
Electrochromic devices usually comprise a pair of glass or plastic plates of which one is mirrored in the case of a mirror. One side of each of these plates is coated with a translucent electrically conductive layer, e.g. indium-tin oxide (ITO). These plates are used to construct a cell by fixing them with their conductively coated sides facing one another. The cell between the plates contains the electrochromic system and is closed tightly. The two plates can be separably connected to a power source and controlled via the conductive layer.
In the electrochromic solution systems known from the above-cited prior art, pairs of redox substances which after reduction or oxidation form coloured, positively or negatively charged free radicals which are chemically reactive are present in a solvent. Examples are the viologen systems which have been known for a long time.
As the pair of redox substances, use is made of one reducible and one oxidizable substance. Both are colourless or have only a slight colour. Under the action of an electric potential, one substance is reduced and the other is oxidized, with at least one becoming coloured. After the potential is switched off, the two original redox substances are formed again, with decolouration or lightening of the colour occurring.
It is known from U.S. Pat. No. 4,902,108 that pairs of redox substances in which the reducible substance has at least two chemically reversible reduction waves in the cyclic voltammogram and the oxidizable substance correspondingly has at least two chemically reversible oxidation waves are suitable. Systems of this type are suitable mainly for dimmable rear view mirrors of automobiles. Since these are solution systems, they are normally not suitable for use in electrochromic windows.
Also known are systems in which the actual electrochromic redox pair is dispersed in a polymer matrix (see, for example, WO-A 96/03475). The undesirable effect of segregation is suppressed in this way.
Combinations of inorganic electrochromic components such as WO.sub.3, NiO or IrO.sub.2 are likewise known and are possibilities as components in an electrochromic window (see, for example, U.S. Pat. No. 5,657,149, Electronique International No. 276, 16 (1997)).
These inorganic electrochromic components can be applied to the conductive substrate only by vapour deposition, sputtering or by a sol-gel technique. As a result, systems of this type are very expensive to produce. Efforts to replace one inorganic component by an organic polymer component have resulted in, for example, electrochromic systems based on the electrically conductive polymer polyaniline (PANI) and WO.sub.3 as complementary electrochromic materials becoming known (see, for example, B. P. Jelle, G. Hagen, J. Electrochem. Soc., Vol. 140, No. 12, 3560 (1993)). An attempt has also been made to use systems without an inorganic component in which the ITO or SnO.sub.2 layer (counterelectrode) is supposed to serve as complementary electrochromic component to substituted poly(3,4-alkylenedioxythiophenes) (U.S. Pat. No. 5,187,608).
However, it is found that such electrochromic assemblies are not able to ensure a sufficient number of switching cycles without a change occurring in the properties of the device.